Power supply apparatus and image forming apparatus

ABSTRACT

The power supply apparatus includes a control unit configured to perform control of gradually changing a turn-on duty of a first switching element when a first voltage mode is switched to a second voltage mode or when the second voltage mode is switched to the first voltage mode.

BACKGROUND OF THE INVENTION Field of the Invention

The present invention relates to a power supply apparatus, for example,a switching power supply that employs an active clamp system in aninsulating convertor using a flyback transformer, and an image formingapparatus including the power supply apparatus.

Description of the Related Art

There has hitherto been a switching power supply configured to convertan AC voltage supplied from, for example, a commercial power source intoa DC voltage. In the switching power supply, in order to reduce powerconsumption, there is a demand for improving the efficiency of theswitching power supply. Here, the efficiency of the switching powersupply refers to a ratio of electric power output from the switchingpower supply with respect to electric power supplied to the switchingpower supply. In Japanese Patent No. 4370844, as a known example of theswitching power supply capable of improving the power supply efficiency,there is disclosed a power supply that employs an active clamp system inan insulating convertor using a flyback transformer.

In Japanese Patent Application Laid-Open No. 2010-142071, as means forimproving the efficiency of the switching power supply in a light loadstate, there is disclosed a method of decreasing an output voltage ofthe switching power supply in the light load state of the switchingpower supply.

However, when the output voltage of the switching power supply ischanged abruptly in the switching power supply employing the activeclamp system, overshoot and undershoot occur. Further, there is also aproblem, for example, in that a through-current flows through aswitching element when the output voltage of the switching power supplyis changed abruptly.

SUMMARY OF THE INVENTION

One aspect of the present invention has been made under theabove-mentioned circumstances, and is a power supply apparatus capableof preventing overshoot and undershoot and preventing a through-currentfrom flowing through a switching element when an output voltage of aswitching power supply is controlled to be changed.

Another aspect of the present invention is a power supply apparatusincluding a transformer including a primary winding and a secondarywinding, a first switching element connected in series to the primarywinding of the transformer, a second switching element connected inparallel to the primary winding of the transformer, a capacitorconnected in series to the second switching element, and is connected inparallel to the primary winding of the transformer together with thesecond switching element, a smoothing and rectifying unit configured tosmooth and rectify a voltage induced to the secondary winding of thetransformer, a feedback unit configured to output a signal according tothe voltage smoothed and rectified by the smoothing and rectifying unit,a control unit configured to control an operation of the first switchingelement and the second switching element based on the signal output fromthe feedback unit so that the voltage smoothed and rectified by thesmoothing and rectifying unit becomes a predetermined voltage, thecontrol unit configured to implement a control operation in a firstvoltage state, in which the predetermined voltage is a first voltage,and in a second voltage state, in which the predetermined voltage is asecond voltage larger than the first voltage, and a notification unitconfigured to notify the control unit of switching between the firstvoltage state and the second voltage state, wherein the control unit isconfigured to perform one of the following: when the notification unitnotifies switching from the first voltage state to the second voltagestate, the control unit changes a condition to a first switchingcondition of performing control of gradually increasing a turn-on dutyof the first switching element, and changes a voltage state to thesecond voltage state after a level of the signal output from thefeedback unit reaches a first value or less, and when the notificationunit notifies switching from the second voltage state to the firstvoltage state, the control unit changes a condition to a secondswitching condition of performing control of gradually decreasing theturn-on duty of the first switching element, and changes a voltage stateto the first voltage state after the level of the signal output from thefeedback unit reaches a second value or more; and when the notificationunit notifies switching from the first voltage state to the secondvoltage state by, the control unit changes a condition to the firstswitching condition of performing control of gradually increasing theturn-on duty of the first switching element, and changes a voltage stateto the second voltage state after the level of the signal output fromthe feedback unit reaches the first value or more, and when thenotification unit notifies switching from the second voltage state tothe first voltage state, the control unit changes a condition to thesecond switching condition of performing control of gradually decreasingthe turn-on duty of the first switching element, and changes a voltagestate to the first voltage state after the level of the signal outputfrom the feedback unit reaches the second value or less.

A further aspect of the present invention is an image forming apparatusincluding an image forming unit configured to form an image, and a powersupply apparatus configured to supply electric power to the imageforming apparatus, the power supply apparatus including a transformerincluding a primary winding and a secondary winding, a first switchingelement connected in series to the primary winding of the transformer, asecond switching element connected in parallel to the primary winding ofthe transformer, a capacitor connected in series to the second switchingelement, and is connected in parallel to the primary winding of thetransformer together with the second switching element, a smoothing andrectifying unit configured to smooth and rectify a voltage induced tothe secondary winding of the transformer, a feedback unit configured tooutput a signal according to the voltage smoothed and rectified by thesmoothing and rectifying unit, a control unit configured to control anoperation of the first switching element and the second switchingelement based on the signal output from the feedback unit so that thevoltage smoothed and rectified by the smoothing and rectifying unitbecomes a predetermined voltage, the control unit configured toimplement a control operation in a first voltage state, in which thepredetermined voltage is a first voltage, and in a second voltage state,in which the predetermined voltage is a second voltage larger than thefirst voltage, and a notification unit, which is configured to notifythe control unit of switching between the first voltage state and thesecond voltage state, wherein the control unit is configured to performone of the following: when the notification unit notifies switching fromthe first voltage state to the second voltage state, the control unitchanges a condition to a first switching condition of performing controlof gradually increasing a turn-on duty of the first switching element,and changes a voltage state to the second voltage state after a level ofthe signal output from the feedback unit reaches a first value or less,and when the notification unit notifies switching from the secondvoltage state to the first voltage state, the control unit changes acondition to a second switching condition of performing control ofgradually decreasing the turn-on duty of the first switching element,and changes a voltage state to the first voltage state after the levelof the signal output from the feedback unit reaches a second value ormore; and when the notification unit notifies switching from the firstvoltage state to the second voltage state, the control unit changes acondition to the first switching condition of performing control ofgradually increasing the turn-on duty of the first switching element,and changes a voltage state to the second voltage state after the levelof the signal output from the feedback unit reaches the first value ormore, and when the notification unit notifies switching from the secondvoltage state to the first voltage state, the control unit changes acondition to the second switching condition of performing control ofgradually decreasing the turn-on duty of the first switching element,and changes a voltage state to the first voltage state after the levelof the signal output from the feedback unit reaches the second value orless.

A further aspect of the present invention is a power supply apparatusincluding a power supply apparatus including a transformer including aprimary winding and a secondary winding, a first switching elementconnected in series to the primary winding of the transformer, a secondswitching element connected in parallel to the primary winding of thetransformer, a capacitor connected in series to the second switchingelement, and connected in parallel to the primary winding of thetransformer together with the second switching element, a smoothing andrectifying unit configured to smooth and rectify a voltage induced tothe secondary winding of the transformer, a feedback unit configured tooutput a signal according to the voltage smoothed and rectified by thesmoothing and rectifying unit, and a control unit configured to controlan operation of the first switching element and the second switchingelement based on the signal output from the feedback unit so that thevoltage smoothed and rectified by the smoothing and rectifying unit is apredetermined voltage, wherein the control is configured to implement acontrol operation in a first voltage state, in which the predeterminedvoltage is a first voltage, and in a second voltage state, in which thepredetermined voltage is a second voltage larger than the first voltage,and wherein the control unit is configured to perform control ofgradually changing a turn-on duty of the first switching element in oneof a case in which the first voltage state is switched to the secondvoltage state and a case in which the second voltage state is switchedto the first voltage state.

A still further aspect of the present invention is an image formingapparatus including an image forming apparatus including an imageforming unit configured to form an image, and a power supply apparatusconfigured to supply electric power to the image forming apparatus, thepower supply apparatus including a transformer including a primarywinding and a secondary winding, a first switching element connected inseries to the primary winding of the transformer, a second switchingelement connected in parallel to the primary winding of the transformer,a capacitor connected in series to the second switching element, andconnected in parallel to the primary winding of the transformer togetherwith the second switching element, a smoothing and rectifying unitconfigured to smooth and rectify a voltage induced to the secondarywinding of the transformer, a feedback unit configured to output asignal according to the voltage smoothed and rectified by the smoothingand rectifying unit, and a control unit configured to control anoperation of the first switching element and the second switchingelement based on the signal output from the feedback unit so that thevoltage smoothed and rectified by the smoothing and rectifying unit is apredetermined voltage, wherein the control unit is configured toimplement a control operation in a first voltage state, in which thepredetermined voltage is a first voltage, and in a second voltage state,in which the predetermined voltage is a second voltage larger than thefirst voltage, and wherein the control unit is configured to performcontrol of gradually changing a turn-on duty of the first switchingelement in one of a case in which the first voltage state is switched tothe second voltage state and a case in which the second voltage state isswitched to the first voltage state.

Further features of the present invention will become apparent from thefollowing description of exemplary embodiments with reference to theattached drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit diagram of a switching power supply in a firstembodiment of the present invention.

FIG. 2A and FIG. 2B are an explanatory graph and an explanatory chart ofa control method for the switching power supply in the first embodiment.

FIG. 3A and FIG. 3B are each an explanatory diagram for illustrating astate in which a through-current flows through the switching powersupply in the first embodiment.

FIG. 4 is a state transition diagram of the switching power supply inthe first embodiment.

FIG. 5A and FIG. 5B are each an explanatory graph of a switching controlmode of an output voltage in the first embodiment.

FIG. 6 is a flowchart for illustrating switching control of the outputvoltage in the first embodiment.

FIG. 7A and FIG. 7B are an explanatory graph and an explanatory chart ofa control method for a switching power supply in a second embodiment ofthe present invention.

FIG. 8A and FIG. 8B are each an explanatory graph of a switching controlmode of an output voltage in the second embodiment.

FIG. 9 is a flowchart for illustrating switching control of the outputvoltage in the second embodiment.

FIG. 10 is a view for illustrating a configuration of an image formingapparatus according to a third embodiment of the present invention.

DESCRIPTION OF THE EMBODIMENTS

Modes for carrying out the present invention are described below indetail based on embodiments with reference to the drawings.

[Power Supply Apparatus]

FIG. 1 is a schematic diagram of a switching power supply 100, which isa power supply apparatus employing an active clamp system, of a firstembodiment of the present invention. An AC voltage Vac (not shown) isinput to the switching power supply 100 from an AC power supply (notshown), for example, a commercial power supply, and is rectified by afull-wave rectifier (for example, bridge diode) (not shown). A DCvoltage Vin is input to a smoothing capacitor C3 of the switching powersupply 100. The switching power supply 100 is an insulating DC/DCconvertor configured to supply a DC voltage stabilized from the voltageVin input to the electrolytic capacitor as the smoothing capacitor C3 ona primary side to a voltage Vout on a secondary side. A potential on alower side of the smoothing capacitor C3 is represented by DCL, and apotential on a higher side thereof is represented by DCH.

The switching power supply 100 includes an insulating transformer T1,which includes a primary winding P1 and an auxiliary winding P2 on theprimary side and a secondary winding S1 on the secondary side. Energy issupplied from the primary winding P1 of the transformer T1 to thesecondary winding S1 thereof by switching control described in FIG. 2Aand FIG. 2B. The auxiliary winding P2 of the transformer T1 is used forsmoothing and rectifying a forward voltage of the voltage Vin applied tothe primary winding P1 with a diode D4 and a capacitor C4 to supply apower supply voltage V1.

On the primary side of the switching power supply 100, there is a fieldeffect transistor FET1 (hereinafter simply referred to as “FET1”), whichis a first switching element connected in series to the primary windingP1 of the transformer T1. Further, on the primary side of the switchingpower supply 100, a circuit in which a capacitor C2 for voltage clampingand an FET2, which is a second switching element, are connected inseries to each other is connected in parallel to the primary winding P1of the transformer T1. Further, on the primary side of the switchingpower supply 100, a control unit 101, which is a control unit for theFET1 and the FET2, and an FET drive unit 102 are arranged. A capacitorC1 for voltage resonance connected in parallel to the FET1 is arrangedin order to reduce a loss at a time of switch-off of the FET1 and theFET2. A capacitance between a drain terminal and a source terminal ofthe FET1 may be used without arranging the capacitor C1 for voltageresonance. A diode D1 in the first embodiment is a body diode of theFET1. Similarly, a diode D2 is a body diode of the FET2.

On the secondary side of the switching power supply 100, a diode D11 anda capacitor C11 are arranged as a secondary-side smoothing andrectifying unit of a flyback voltage induced to the secondary winding S1of the transformer T1. On the secondary side of the switching powersupply 100, there is a feedback unit 116 serving as a feedback unit tobe used for feeding back the voltage Vout output to the secondary sideto the primary side. The feedback unit 116 outputs a signal according tothe voltage Vout to the control unit 101.

The feedback unit 116 is used for controlling the voltage Vout to twoconstant voltages in a case in which the voltage Vout to be outputserves as a first voltage of 5 V (first voltage mode) and a case inwhich the voltage Vout to be output serves as a second voltage of 24 V,which is larger than the first voltage (second voltage mode). Thecontrol unit 101 determines a turn-on duty of the FET1 and a cycle of aswitching operation based on a signal output from the feedback unit 116.When the level of the signal from the feedback unit 116 is the same, thecontrol unit 101 controls the turn-on duty of the FET1 in the firstvoltage mode to be lower than that of the FET1 in the second voltagemode. A voltage value of the voltage Vout is set based on a referencevoltage of a reference terminal REF of a shunt regulator IC5 andresistances of a resistor R52, a resistor R54, and a resistor R55. Whenthe voltage Vout becomes high, a current of a cathode terminal K of theshunt regulator IC5 increases. Therefore, a current of a secondary-sidediode of a photocoupler PC5 flowing through a resistor R51 increases.Then, a primary-side transistor of the photocoupler PC5 is operated, anda discharge amount of charge from a capacitor C6 increases. Therefore,an FB terminal voltage of the control unit 101 decreases. Further, whenthe voltage of the voltage Vout becomes low, the discharge amount ofcharge from the capacitor C6 decreases. Therefore, the FB terminalvoltage of the control unit 101 increases due to a current flowing froma power supply voltage V2 to the capacitor C6 through a resistor R2. Thecontrol unit 101 performs feedback control for controlling the voltageVout to the predetermined constant voltages by detecting the FB terminalvoltage. Further, in the feedback unit 116, when a STANBY signal to beinput reaches a high-level state, an FET51 is turned on, and theresistor R55 is short-circuited. Therefore, a target voltage to besubjected to feedback control increases, and the feedback unit 116 isbrought into a state of outputting a voltage of 24 V to the voltageVout. A resistor R56 is connected between a gate terminal and a sourceterminal of the FET 51. When the STANBY signal reaches a low-levelstate, the FET 51 is turned off, and the resistors R54 and R55 areconnected in series to each other. Therefore, the target voltage to besubjected to feedback control decreases, and the feedback unit 116 isbrought into a state of outputting a voltage of 5 V to the voltage Vout.

In the first embodiment, description is given of a case in which, whenthe voltage of the voltage Vout becomes low, the FB terminal voltageincreases, and when the voltage of the voltage Vout becomes high, the FBterminal voltage decreases. However, it is also possible to design thefeedback unit 116 conversely so that, when the voltage of the voltageVout becomes low, the FB terminal voltage decreases, and when thevoltage of the voltage Vout becomes high, the FB terminal voltageincreases. A switching control method for an output voltage of thepresent invention is also effective when the operation of the FBterminal voltage with respect to the voltage of the voltage Vout isopposite.

A switching notification unit 118, which is a notification unit, is usedfor notifying the control unit 101 of timing for switching between thefirst voltage mode corresponding to a first voltage state and the secondvoltage mode corresponding to a second voltage state, based on theSTANBY signal to be input. When the STANBY signal reaches a high-levelstate, an FET81 is turned on, and a current flows to a secondary-sidediode of a photocoupler PC8 through a resistor R81. After that, when aprimary-side transistor of the photocoupler PC8 is operated, charge isdischarged from a capacitor C8, and a low-level state of a voltage of anSL terminal of the control unit 101 can be detected. A resistor R82 is aresistor between a gate and a source of the FET81.

Meanwhile, when the STANBY signal reaches a low-level state, thecapacitor C8 is charged from the power supply voltage V2 described laterthrough a resistor R1, and a high-level state of the voltage of the SLterminal of the control unit 101 can be detected. The control unit 101determines switching of an output voltage between the first voltage mode(SL terminal=high-level state) of outputting 5 V to the voltage Vout andthe second voltage mode (SL terminal=low-level state) of outputting 24 Vto the voltage Vout, based on the voltage of the SL terminal.

In the first embodiment, as the control unit 101, a calculation controlunit (for example, a CPU or an ASIC) that is operated with a clockgenerated by a transmitter is used. Through use of the calculationcontrol unit that is operated with a clock as the control unit 101, acomplicated control sequence as illustrated in FIG. 4 to FIG. 6 in thefirst embodiment can be implemented with a simple and inexpensivecircuit configuration.

The power supply voltage V2, which is generated by a DC/DC convertor104, is supplied between a VC terminal and a G terminal of the controlunit 101. The control unit 101 outputs a control signal DS1 (drivesignal of the FET1) and a control signal DS2 (drive signal of the FET2)based on a voltage signal input to an FB terminal, and controls the FET1and the FET2 through the FET drive unit 102. Further, a VS terminal ofthe control unit 101 is a terminal to be used for detecting the voltageVin input to the smoothing capacitor C3 by dividing the power supplyvoltage V1 obtained by smoothing and rectifying the forward voltagegenerated in the auxiliary winding P2 of the transformer T1 with theresistors R3 and R4 and detecting the resultant power supply voltage V1.The SL terminal and the FB terminal are described later.

The FET drive unit 102 is a circuit configured to generate a gate drivesignal DL of the FET1 according to the control signal DS1 and a gatedrive signal DH of the FET2 according to the control signal DS2. Thepower supply voltage V1 is supplied between a VC terminal and a Gterminal of the FET drive unit 102. Further, in order to drive the FET2,the power supply voltage V1 is supplied between a VH terminal and a GHterminal of the FET drive unit 102 by a charge pump circuit formed of acapacitor C5 and a diode D5. When the control signal DS1 reaches ahigh-level state, the FET drive unit 102 sets the gate drive signal DLof the FET1 to a high-level state, and the FET1 is turned on. Similarly,when the control signal DS2 reaches a high-level state, the FET driveunit 102 sets the gate drive signal DH of the FET2 to a high-levelstate, and the FET2 is turned on.

The DC/DC convertor 104 is a three-terminal regulator or a step-downswitching power supply, and outputs the power supply voltage V2 from thepower supply voltage V1 input between a VC terminal and a G terminal toan OUT terminal. A start-up circuit 103 is a three-terminal regulator ora step-down switching power supply, and outputs the power supply voltageV1 from the power supply voltage Vin input between a VC terminal and a Gterminal to an OUT terminal. The start-up circuit 103 is a circuit thatis operated only when the power supply voltage V1 supplied from theauxiliary winding P2 has a predetermined voltage value or less, and isused for supplying the power supply voltage V1 at a time of start-up ofthe switching power supply 100.

Further, the control unit 101 can grasp the state of a load of thevoltage Vout by monitoring the FB terminal voltage, and hence canperform appropriate control in accordance with the state of the load.Therefore, the control unit 101 functions as a detection unit configuredto detect the state of the load. In order to allow the control unit 101to determine the state of the load of the voltage Vout more accurately,a current detection unit (not shown) may be arranged in a path forsupplying electric power to the FET1 and the load of the switching powersupply 100.

[Relationship Between FB Terminal Voltage and Turn-on Duty of FET1]

FIG. 2A and FIG. 2B are an explanatory graph and an explanatory chart ofa control method for the FET1 and the FET2 by the control unit 101. FIG.2A is a graph of a relationship between the FB terminal voltage and theturn-on duty of the FET1 in the first voltage mode (at a time of outputof 5 V, represented by the dotted line) and the second voltage mode (ata time of output of 24 V, represented by the solid line). Further, thefirst voltage mode includes a case in which the AC voltage Vac has aneffective value of 110 Vrms (voltage Vin=155 V) and a case in which theAC voltage Vac has an effective value of 120 Vrms (voltage Vin=170 V).Further, the second voltage mode includes a case in which the AC voltageVac has an effective value of 110 Vrms (voltage Vin=155 V) and a case inwhich the AC voltage Vac has an effective value of 120 Vrms (voltageVin=170 V). The turn-on duty refers to a ratio of a turn-on time periodwith respect to one cycle during which the FET is turned on or off.

The control unit 101 performs control of increasing the turn-on duty ofthe FET1 when the FB terminal voltage increases in any of the firstvoltage mode and the second voltage mode. Further, the voltage value ofthe voltage Vout is significantly different between the first voltagemode and the second voltage mode, and hence the control unit 101performs control of increasing the turn-on duty of the FET1 in thesecond voltage mode as compared to that in the first voltage mode evenat the same FB terminal voltage. When the relationship between the FBterminal voltage and the turn-on duty of the FET1 is optimally set ineach of the first voltage mode and the second voltage mode, theresolution of the FB terminal voltage with respect to the voltage Voutcan be enhanced. Further, the control unit 101 sets the relationshipbetween the FB terminal voltage and the turn-on duty of the FET1 in eachof the first voltage mode and the second voltage mode based on thevoltage value of the voltage Vin detected at the VS terminal. Thus, evenwhen the voltage Vin is different, the turn-on duty of the FET1 can beoptimally controlled. FIG. 2A is a graph of a relationship between theFB terminal voltage and the turn-on duty of the FET1 at a time when theAC voltage Vac is 110 Vrms and 120 Vrms. Specifically, when the voltagevalue of the AC voltage Vac is high, the control unit 101 performscontrol of decreasing the turn-on duty of the FET1 as compared to a casein which the voltage value of the AC voltage Vac is low, by setting theturn-on time period of the FET1 to be short.

In FIG. 2B, a switching operation using the FET1 and the FET2 isdescribed. In (i) of FIG. 2B, there is illustrated a waveform of a gatedrive voltage of the FET1, that is, the control signal DS1, and in (ii)of FIG. 2B, there is illustrated a waveform of a gate drive voltage ofthe FET2, that is, the control signal DS2. In (iii) of FIG. 2B, there isillustrated a waveform of a current flowing through the drain terminalof the FET1, and in (iv) of FIG. 2B, there is illustrated a waveform ofa voltage between the drain terminal and the source terminal of theFET1. The control unit 101 repeatedly controls the turn-on time periodof the FET1 and the turn-on time period of the FET2 by alternatelyturning on or off the FET1 and the FET2 with a dead time beinginterposed. An operation of alternately turning on or off the FET1 andthe FET2 repeatedly with a dead time being interposed is referred to asa continuous operation. Further, the dead time refers to a time periodduring which neither of the FET1 and the FET2 is turned on. The waveformof the control signal DS1 represents a waveform of the gate drivevoltage of the FET1, and the waveform of the control signal DS2represents a waveform of the gate drive voltage of the FET2.

As illustrated in FIG. 2B, during the turn-on time period of the FET1, acurrent flows between the drain terminal and the source terminal of theFET1, and energy is supplied to the transformer T1. It is required toincrease the energy supplied to the transformer T1 by elongating theturn-on time period of the FET1 as the load of the voltage Voutincreases.

During the turn-on time period of the FET2, the energy is supplied fromthe secondary winding S1 of the transformer T1 to the circuit on thesecondary side. Then, together with this, as illustrated in FIG. 2B,through a resonant operation of the capacitor C2 for voltage clampingand the transformer T1, a resonant voltage waveform is applied betweenthe drain terminal and the source terminal of the FET1. It is desiredthat the turn-on time period of the FET2 be appropriately set inaccordance with the voltage value and the load of the voltage Vout. Asthe load of the voltage Vout increases, a time period required forsupplying the energy from the secondary winding S1 of the transformer T1to the circuit on the secondary side becomes longer. Therefore, it isrequired to set the turn-on time period of the FET2 to be long. Further,when the voltage value of the voltage Vout is low, the voltage of thesecondary winding S1 decreases, and a time period required for supplyingthe energy to the circuit on the secondary side becomes long. Therefore,it is required to set the turn-on time period of the FET2 to be long.For this reason, when the load of the voltage Vout is heavy or when thevoltage value of the voltage Vout is low, the control unit 101 performscontrol of elongating the turn-on time period of the FET2.

TABLE 1 Turn-on duty Turn-on time Turn-on time Vout of FET1 period ofFET1 period of FET2 First voltage  5 V Small Short Long mode Secondvoltage 24 V Large Long Short mode Comparison at FB terminal voltage of1.5 V and Vac = 110 Vrms (Vin = 155 V)

In Table 1, the above-mentioned features of switching control in thefirst voltage mode and the second voltage mode are summarized. In Table1, comparison of switching control is made between the first voltagemode and the second voltage mode under a condition in which the FBterminal voltage and the VS terminal voltage are the same. As anexample, there is given a case in which the FB terminal voltage is setto 1.5 V, and the AC voltage Vac has an effective value of 110 Vrms(voltage Vin=155 V). In the second voltage mode, the switching powersupply 100 is in a standby state, and the load of the voltage Voutbasically increases. Thus, in order to supply larger energy to thetransformer T1, the turn-on time period of the FET1 is set to be longerthan that in the first voltage mode.

As described above, in the first voltage mode, the voltage value of thevoltage Vout is about ⅕ as compared to that of the second voltage mode.When the turn-on time period of the FET1 is the same, it is required toelongate the turn-on time period of the FET2 by about 5 times in thefirst voltage mode as compared to that of the second voltage mode.Therefore, in the first voltage mode, the turn-on time period of theFET2 is set to be longer than that in the second voltage mode. Further,as shown in FIG. 2A, the turn-on duty of the FET1 in the first voltagemode is smaller than that in the second voltage mode. The features shownin Table 1 are examples for describing the control sequence example ofFIG. 5A, FIG. 5B, and FIG. 6, and the switching control method for anoutput voltage of the present invention is also applicable to a case towhich the features of Table 1 do not apply.

As described in FIG. 2A and FIG. 2B, the turn-on duty of the FET1 andthe like are significantly different between the first voltage mode andthe second voltage mode. Therefore, when switching control of the firstvoltage mode and the second voltage mode is performed, the followingproblems occur. That is, when the turn-on time period of the FET1 andthe turn-on time period of the FET2 are changed abruptly with the samegain as that of normal feedback control, overshoot and undershoot occurin the voltage Vout. Further, the switching control becomes unstable,and there is a risk in that a through-current may flow during a reverserecovery time of the diode described in FIG. 3A and FIG. 3B.

[Regarding Through-Current]

In FIG. 3A and FIG. 3B, a through-current during a reverse recovery timeof the body diodes of the FET1 and the FET2 is illustrated through useof schematic diagrams for illustrating main portions of the switchingpower supply 100 of FIG. 1. In the left figure of FIG. 3A, both the FET1and the FET2 are in a non-conductive state, and the body diode D2 of theFET2 is in a conductive state. The right figure of FIG. 3A is anexplanatory diagram for illustrating a case in which the FET1 is turnedon during conduction of the body diode D2 of the FET2, and athrough-current flows during a reverse recovery time of the body diodeD2.

Similarly, in the left figure of FIG. 3B, both the FET1 and the FET2 arein a non-conductive state, and the body diode D1 of the FET1 is in aconductive state. The right figure of FIG. 3B is an explanatory diagramfor illustrating a case in which the FET2 is turned on during conductionof the body diode D1 of the FET1, and a through-current flows during areverse recovery time of the body diode D1.

As described above, when a through-current flows during a reverserecovery time of the diode, an abrupt current change occurs, and theFET1 and the FET2 malfunction, with the result that there is a risk inthat the switching power supply 100 may have a failure. The firstembodiment has an object of preventing overshoot and undershoot of thevoltage Vout and preventing a switching condition in which athrough-current described in FIG. 3A and FIG. 3B flows by performingswitching control of the voltage Vout described in FIG. 4 to FIG. 6.

[State Transition of Switching Power Supply]

FIG. 4 is a state transition diagram of the switching power supply 100in the switching control of the voltage Vout. The control unit 101 setsthe turn-on duty of the FET1 based on the FB terminal voltage in thefirst voltage mode to output the low voltage Vout (for example, 5 V).When the control unit 101 detects that the STANBY signal is in ahigh-level state in the first voltage mode [1], the control unit 101transitions to a switching control mode 1, which is a first switchingcondition, in order to change a mode from the first voltage mode to thesecond voltage mode.

FIG. 5A is a graph for showing a state in which the control unit 101changes a mode from the first voltage mode to the second voltage modethrough the switching control mode 1 in order to change a mode from thefirst voltage mode to the second voltage mode. The horizontal axis ofFIG. 5A represents a time, and also represents a period of each mode.Further, the vertical axis of FIG. 5A represents the turn-on duty of theFET1 with the solid line and the FB terminal voltage with the brokenline. As shown in FIG. 5A, in the switching control mode 1, the controlunit 101 gradually increases the turn-on duty of the FET1 at asufficiently low speed to increase the voltage Vout. In this case, thefeedback unit 116 is in a state of outputting an output of 24 V (secondvoltage mode). Therefore, the voltage Vout is in a state of insufficientvoltage, and the FB terminal voltage increases to a maximum value. It isunderstood that the FB terminal voltage represented by the broken linein FIG. 5A reaches a maximum value during a period of the switchingcontrol mode 1.

When the voltage Vout reaches the voltage of the second voltage mode,the FB terminal voltage starts decreasing. When the FB terminal voltagereaches Vth24 (first value) or less, the control unit 101 transitionsfrom the switching control mode 1 to the second voltage mode [2]. Thecontrol unit 101 sets the turn-on duty of the FET1 based on the FBterminal voltage in the second voltage mode to output the high voltageVout (for example, 24 V). When the control unit 101 detects that theSTANBY signal is in a low-level state in the second voltage mode [3],the control unit 101 transitions to a switching control mode 2, which isa second switching condition, in order to change a mode from the secondvoltage mode to the first voltage mode.

FIG. 5B is a graph for showing a state in which the control unit 101changes a mode from the second voltage mode to the first voltage modethrough the switching control mode 2 in order to change a mode from thesecond voltage mode to the first voltage mode. The horizontal axis ofFIG. 5B represents a time, and also represents a period of each mode.The vertical axis of FIG. 5B represents the turn-on duty of the FET1 andthe FB terminal voltage. As shown in FIG. 5B, in the switching controlmode 2, the control unit 101 gradually decreases the turn-on duty of theFET1 at a sufficiently low speed to decrease the voltage Vout. In thiscase, the feedback unit 116 is in a state of outputting an output of 5 V(first voltage mode). Therefore, the voltage Vout is in a state ofexcessive voltage, and the FB terminal voltage decreases to a lowerlimit value. It is understood that the FB terminal voltage representedby the broken line in FIG. 5B reaches a minimum value during a period ofthe switching control mode 2. When the voltage Vout decreases to thevoltage of the first voltage mode, the FB terminal voltage startsincreasing. When the FB terminal voltage reaches Vth5 (second value) ormore, the control unit 101 transitions from the switching control mode 2to the first voltage mode [4].

Further, when the control unit 101 detects that the STANBY signal is ina low-level state during control of the switching control mode 1 [5],the control unit 101 changes a mode to the switching control mode 2.Similarly, when the control unit 101 detects that the STANBY signal isin a high-level state during control of the switching control mode 2[6], the control unit 101 changes a mode to the switching control mode1. Through this control, the control unit 101 can also support the casein which the STANBY signal changes in each of the switching controlmodes.

Further, in the switching control mode 1 and the switching control mode2, a speed at which the turn-on duty of the FET1 is changed may be madevariable based on the above-mentioned state of the load detected by thecontrol unit 101, which is a detection unit of the load of the voltageVout. For example, in the switching control mode 2, a time period duringwhich charge of the capacitor C11 is discharged varies depending on theload of the voltage Vout. Therefore, when the load of the voltage Voutis heavy, the speed at which the turn-on duty of the FET1 is changed inthe switching control mode 2 may be controlled to increase. Similarly,in the switching control mode 1, it is required to supply electric powerto the load of the voltage Vout in addition to electric power requiredfor charging the capacitor C11. Therefore, when the load of the voltageVout is heavy, the speed at which the turn-on duty of the FET1 ischanged in the switching control mode 1 may be controlled to decrease.

Further, in the switching control mode 1 and the switching control mode2, the speed at which the turn-on duty of the FET1 is changed may not bea constant speed. For example, in order to reduce a time period to betaken in each of the switching control modes while preventing overshootand undershoot of the voltage Vout, the following control may beperformed. That is, the speed at which the turn-on duty of the FET1 ischanged may be set to be high in a former half of the switching controlmode 1 and the switching control mode 2, and the speed at which theturn-on duty of the FET1 is changed may be set to be low in a latterhalf thereof.

As described above, in the first embodiment, the switching notificationunit 118 notifies the control unit 101 of switching between the firstvoltage mode and the second voltage mode with the STANBY signal, andperforms the switching control sequence described in FIG. 4, FIG. 5A,and FIG. 5B. With this, an abrupt change in the voltage Vout and afluctuation in switching waveform can be prevented, and it is possibleto prevent overshoot and undershoot of the voltage Vout and theswitching condition in which a through-current flows described in FIG.3A and FIG. 3B.

[Control Processing of Switching Power Supply]

FIG. 6 is a flowchart for illustrating a control sequence of theswitching power supply 100 by the control unit 101 in the firstembodiment. When the switching power supply 100 is supplied with thevoltage Vin, the control unit 101 starts control in Step (hereinafterabbreviated as “S”) 101 and the subsequent steps. In S101, the controlunit 101 controls the FET1 based on the turn-on duty of the FET1according to the FB terminal voltage in the first voltage mode, andcontinues the state of outputting 5 V to the voltage Vout. In S102, thecontrol unit 101 determines whether or not the STANBY signal suppliedfrom outside of the switching power supply 100 is in a high-level state.When the control unit 101 determines in S102 that the STANBY signal isin a high-level state, the control unit 101 advances the flow to S103,and transitions from the first voltage mode to the switching controlmode 1. When the control unit 101 determines in S102 that the STANBYsignal is not in a high-level state, that is, the STANBY signal is in alow-level state, the control unit 101 returns the flow to S101, andcontinues the first voltage mode.

In S103, the control unit 101 performs control of gradually increasingthe turn-on duty of the FET1 in the switching control mode 1. In S104,the control unit 101 determines whether or not the STANBY signalsupplied from outside of the switching power supply 100 is in alow-level state. When the control unit 101 determines in S104 that theSTANBY signal is in a low-level state, the control unit 101 advances theflow to S108, and transitions from the switching control mode 1 to theswitching control mode 2. When the control unit 101 determines in S104that the STANBY signal is not in a low-level state, that is, the STANBYsignal is in a high-level state, the control unit 101 advances the flowto S105.

In S105, the control unit 101 determines whether or not the FB terminalvoltage has decreased to Vth24 as a first value stored in a memory (notshown) in the control unit 101, or less. When the operation of the FBterminal voltage is opposite, the control unit 101 determines whether ornot the FB terminal voltage has increased to Vth24 (the first value) ormore. When the control unit 101 determines in S105 that the FB terminalvoltage is Vth24 or less, the control unit 101 advances the flow toS106, and transitions from the switching control mode 1 to the secondvoltage mode. When the control unit 101 determines in S105 that the FBterminal voltage is larger than Vth24, the control unit 101 returns theflow to S103, and continues the switching control mode 1.

In S106, the control unit 101 controls the FET1 based on the turn-onduty of the FET1 according to the FB terminal voltage in the secondvoltage mode, and continues the state of outputting 24 V to the voltageVout. In S107, the control unit 101 determines whether or not the STANBYsignal supplied from outside of the switching power supply 100 is in alow-level state. When the control unit 101 determines in S107 that theSTANBY signal is in a low-level state, the control unit 101 advances theflow to S108, and transitions from the second voltage mode to theswitching control mode 2. When the control unit 101 determines in S107that the STANBY signal is not in a low-level state, that is, the STANBYsignal is in a high-level state, the control unit 101 returns the flowto S106, and continues the second voltage mode.

In S108, the control unit 101 performs control of gradually decreasingthe turn-on duty of the FET1 in the switching control mode 2. In S109,the control unit 101 determines whether or not the STANBY signalsupplied from outside of the switching power supply 100 is in ahigh-level state. When the control unit 101 determines in S109 that theSTANBY signal is in a high-level state, the control unit 101 returns theflow to S103, and transitions from the switching control mode 2 to theswitching control mode 1. When the control unit 101 determines in S109that the STANBY signal is not in a high-level state, that is, the STANBYsignal is in a low-level state, the control unit 101 advances the flowto S110.

In S110, the control unit 101 determines whether or not the FB terminalvoltage has increased to Vth5 as the second value) stored in the memory(not shown) in the control unit 101, or more. When the operation of theFB terminal voltage is opposite, the control unit 101 determines whetheror not the FB terminal voltage has decreased to Vth5 (second value) orless. When the control unit 101 determines in S110 that the FB terminalvoltage is Vth5 or more, the control unit 101 returns the flow to S101,and transitions from the switching control mode 2 to the first voltagemode. When the control unit 101 determines in S110 that the FB terminalvoltage is smaller than Vth5, the control unit 101 returns the flow toS108, and continues the switching control mode 2. The control unit 101performs switching control of the voltage Vout of the switching powersupply 100 by repeating the control in the above-mentioned steps S101 toS110.

In the first embodiment, description is given under the condition inwhich the threshold values Vth5 and Vth24 of the FB terminal voltage arefixed values. However, the threshold values Vth5 and Vth24 may bevariable values according to the voltage value of the input voltage Vinand the load to the output voltage Vout.

The control method for the switching power supply 100 in the firstembodiment has the following features.

-   -   The control unit 101 has the first voltage mode to output a low        voltage and the second voltage mode to output a high voltage.    -   The switching power supply 100 includes the switching        notification unit 118 for switching between the first voltage        mode and the second voltage mode.    -   The control unit 101 has the switching control mode for        performing transition between the first voltage mode and the        second voltage mode (the control unit 101 invalidates the        control by the FB terminal voltage, and performs control of        gradually increasing or decreasing the turn-on duty of the        FET1.)    -   The control unit 101 ends the switching control mode when the FB        terminal voltage reaches a predetermined voltage value or more        or when the FB terminal voltage reaches a predetermined voltage        value or less, and transitions to the first voltage mode or the        second voltage mode.

Through use of the above-mentioned control method in the firstembodiment, overshoot and undershoot of the output from the switchingpower supply can be prevented, and further a through-current can beprevented from flowing through the switching element, with the resultthat the reliability of the switching power supply can be enhanced. Asdescribed above, according to the first embodiment, when the outputvoltage of the switching power supply is controlled to be changed,overshoot and undershoot can be prevented, and a through-current can beprevented from flowing through the switching element.

Next, a control method for the switching power supply 100 of a secondembodiment of the present invention is described. The sameconfigurations as those of the first embodiment are denoted by the samereference symbols, and description thereof is omitted. The controlmethod for the switching power supply 100 in the second embodimentdescribed with reference to FIG. 7A to FIG. 9 is different from that inthe first embodiment in that, when the load of the voltage Vout is lowin the first voltage mode, intermittent control is performed in order toimprove power supply efficiency. Further, the control method for theswitching power supply 100 in the second embodiment is different fromthat in the first embodiment in that, there is Step 1, which is firstcontrol of elongating a cycle in the switching control mode 1, and thatthere is a prohibition period of the intermittent control in theswitching control mode 2.

[Control Method for FET1 and FET2]

FIG. 7A and FIG. 7B are an explanatory graph and an explanatory chart ofthe control method for the FET1 and the FET2 by the control unit 101. InFIG. 7A, the horizontal axis represents the FB terminal voltage, and thevertical axis represents the turn-on duty of the FET, with the solidline indicating the case in which a voltage Vout1 is 24 V and the brokenline indicating the case in which the voltage Vout1 is 5 V. Further, inthe same manner as in FIG. 2A, FIG. 7A is a graph for showing the casein which the input voltage Vin is different in each of the modes. InFIG. 7A, when the FB terminal voltage decreases to a voltage value FBL1,which is a first level stored in the memory (not shown) included in thecontrol unit 101, in the first voltage mode (at a time of output of 5 V,represented by the broken line), intermittent control described in FIG.7B is performed. This point is different from the control method in thefirst embodiment. When the load of the voltage Vout is light in thefirst voltage mode, by performing the intermittent control described inFIG. 7B, a switching loss of the FET1 and the FET2 is reduced and thepower supply efficiency of the switching power supply 100 can beimproved.

In FIG. 7B, description is given of control in an intermittent operation(hereinafter referred to as “intermittent control”) of repeating aswitching period, which is a first period, and a halt period, which is asecond period. In FIG. 7B, graphs (i) to (iv) are similar to the graphs(i) to (iv) of FIG. 2B, and hence description thereof is omitted. InFIG. 7B, a graph (v) represents the FB terminal voltage, and the voltagevalue FBL1, which is a first level, and a voltage value FBL2, which is asecond level, are each represented by an alternate long and short dashline.

In a light load state of the switching power supply 100, theintermittent control of repeating the switching period and the haltperiod of halting the switching operation is performed as shown in FIG.7B. With this, the current on the primary side of the switching powersupply 100 and the number of switching of the FET1 and the FET2 can bereduced to improve the power supply efficiency in the light load stateof the switching power supply 100. In the second embodiment, when the FBterminal voltage becomes lower than the voltage value FBL1, it isdetermined that the switching power supply 100 is brought into a lightload state, and movement to the halt period is performed. When the FBterminal voltage becomes larger than the voltage value FBL2, which isthe second level stored in the memory (not shown) included in thecontrol unit 101, after the movement to the halt period, a period ismoved from the halt period to the switching period again. In theswitching power supply 100, the voltage value FBL2 is set to a voltagelarger than the voltage value FBL1 (FBL2>FBL1). Then, the intermittentcontrol shown in FIG. 7B is implemented through use of overshoot andundershoot of the FB terminal voltage. A cycle during which theswitching period and the halt period are repeatedly controlled in thiscase is referred to as an intermittent control cycle. As a method ofdetermining the light load state of the switching power supply 100, asdescribed above, a method of arranging a current detection unit (notshown) in a path for supplying electric power to the FET1 and the loadof the switching power supply 100 may be used.

[Switching Control Mode]

FIG. 8A and FIG. 8B are each an explanatory graph of a switching controlmode of the voltage Vout. When the control unit 101 detects that theSTANBY signal is in a high-level state in the first voltage mode tooutput 5 V to the voltage Vout, the control unit 101 transitions to theswitching control mode 1 in order to change a mode from the firstvoltage mode to the second voltage mode. The switching control mode 1described in the second embodiment is characterized in that the controlmode is divided into two-stage controls, Step 1 as the first control,and Step 2 as the second control, as shown in FIG. 8A and FIG. 8B.

In FIG. 8A and FIG. 8B, the horizontal axis represents a time, and thevertical axis represents the turn-on duty of the FET1 with the thicksolid line, the switching cycle that is the cycle of the switchingoperation with the thin solid line, and the FB terminal voltage with thebroken line. In the switching control mode 1, the voltage Vout isincreased by gradually increasing the turn-on duty of the FET1 at aspeed sufficiently lower than that of normal feedback control. In Step 1of the switching control mode 1, the turn-on duty of the FET1 isincreased by elongating the turn-on time period of the FET1, and henceStep 1 has a feature in that the switching cycle becomes long.Meanwhile, in Step 2 of the switching control mode 1, the turn-on dutyof the FET1 is increased by shortening the turn-on time period of theFET2, and hence the switching cycle becomes short as a whole. As shownin FIG. 8A, the switching cycle becomes long in Step 1, and becomesshort in Step 2.

As an advantage obtained by elongating the switching cycle in Step 1 ofthe switching control mode 1, there is given the following. When theswitching cycle is elongated, a switching waveform suitable for the casein which the load of the voltage Vout is heavy is obtained as describedin the first embodiment. When the load of the voltage Vout is heavy inthe switching control mode 1, switching control that is also suitablefor a heavy load can be performed by performing control in Step 1 of theswitching control mode 1.

In Step 2 of the switching control mode 1, although the switching cyclebecomes short, the voltage value of the voltage Vout increases, andhence the voltage of the secondary winding 51 increases. Therefore, theswitching control mode 1 can also support the case in which the load ofthe voltage Vout is heavy. Accordingly, it is effective to perform thecontrol in Step 1 of elongating the frequency of switching control andthen perform control in Step 2 of shortening the frequency in theswitching control mode 1.

When the control unit 101 detects that the STANBY signal is in alow-level state in the second voltage mode to output 24 V to the voltageVout, the control unit 101 transitions from the second voltage mode tothe switching control mode 2. In the switching control mode 2, thevoltage Vout is decreased by gradually decreasing the turn-on duty ofthe FET1 at a speed sufficiently lower than that of normal feedbackcontrol as shown in FIG. 8B. In FIG. 8B, the horizontal axis representsa time, and the vertical axis represents the turn-on duty of the FET1with the solid line and the FB terminal voltage with the broken line.

In the switching control mode 2 in the second embodiment, the controlunit 101 does not change a control to the intermittent control even whenthe FB terminal voltage decreases to the voltage value FBL1. The controlunit 101 decreases the turn-on duty of the FET1 until the FB terminalvoltage reaches Vth5 or more or the turn-on duty of the FET1 reaches aminimum duty (hereinafter referred to as “MinDuty”) stored in anon-volatile memory (not shown) included in the control unit 101.

Then, the control unit 101 continues a period of prohibiting theintermittent control until the switching control mode 2 is ended. Asshown in FIG. 8B, after the control unit 101 transitions to theswitching control mode 2, the FB terminal voltage becomes lower than thevoltage value FBL1 represented by the alternate long and short dashline. However, the intermittent control is prohibited, and hence theturn-on duty of the FET1 gradually decreases.

The value of MinDuty is a lower limit value of the turn-on duty of theFET1. When the turn-on duty of the FET1 decreases to MinDuty (alternatelong and two short dashes line), which is a predetermined duty, theswitching control mode 2 is ended even when the FB terminal voltage hasnot reached Vth5 or more.

Through setting of the prohibition period of the intermittent control inthe switching control mode 2, the continuous control state described inFIG. 2B can be kept, and the responsiveness of control can be improved.Thus, through setting of the prohibition period of the intermittentcontrol, undershoot of the voltage Vout can be prevented.

[Control Processing of Switching Power Supply]

FIG. 9 is a flowchart for illustrating a control sequence of theswitching power supply 100 by the control unit 101 in the secondembodiment. The same control as that in the flowchart of FIG. 6 isdenoted by the same reference symbol, and description thereof isomitted. When the control unit 101 detects in S102 that the STANBYsignal is in a high-level state, the control unit 101 transitions fromthe first voltage mode to the switching control mode 1, and advances theflow to S201. In S201, the control unit 101 performs the control in Step1 of the switching control mode 1 of gradually elongating the turn-ontime period of the FET1.

In S202, the control unit 101 determines whether or not the turn-on timeperiod of the FET1 has reached FET1_24 (predetermined time) stored inthe non-volatile memory (not shown) in the control unit 101, or more.When the control unit 101 determines in S202 that the turn-on timeperiod of the FET1 has reached FET1_24 or more, the control unit 101advances the flow to S203. When the control unit 101 determines in S202that the turn-on time period of the FET1 is less than FET1_24, thecontrol unit 101 returns the flow to S201, and continues the control inStep 1 of the switching control mode 1. In S203, the control unit 101performs the control in Step 2 of the switching control mode 1 ofgradually shortening the turn-on time period of the FET2.

The processing in S204 to S205 is control for preventing overshoot at atime when the voltage Vout has reached a voltage of 24 V of the secondvoltage mode in Step 2 of the switching control mode 1. In S204, thecontrol unit 101 determines whether or not the FB terminal voltage hasreached Vth24 or less. When the control unit 101 determines in S204 thatthe FB terminal voltage has reached Vth24 or less, the control unit 101advances the flow to S205. In S205, the control unit 101 resumes thecontrol of the FET1 based on the turn-on duty of the FET1 according tothe FB terminal voltage. When the control unit 101 determines in S204that the FB terminal voltage is larger than Vth24, the control unit 101advances the flow to S206.

In S206, the control unit 101 determines whether or not the turn-on timeperiod of the FET2 has reached FET2_24 stored in the non-volatile memory(not shown) in the control unit 101, or less. When the control unit 101determines in S206 that the turn-on time period of the FET2 has reachedFET2_24 or less, the control unit 101 advances the flow to S207. Whenthe control unit 101 determines in S206 that the turn-on time period ofthe FET2 is larger than FET2_24, the control unit 101 returns the flowto S203, and continues the control in Step 2 of the switching controlmode 1. In S207, the control unit 101 determines whether or not thecontrol of the FET1 based on the turn-on duty of the FET1 according tothe FB terminal voltage has been resumed in S205. When the control ofthe FB terminal voltage by the control unit 101 has been resumed, thecontrol unit 101 advances the flow to S106, and transitions from theswitching control mode 1 to the second voltage mode.

The processing in S208 is the control in a case in which the voltageVout has not reached a voltage of 24 V of the second voltage mode aftercompletion of Step 2 of the switching control mode 1. In S208, thecontrol unit 101 performs control of elongating the turn-on time periodof the FET1 until the FB terminal voltage satisfies the condition inwhich the FB terminal voltage is Vth24 or less, and advances theprocessing to S106 to transit the operation mode from the switchingcontrol mode 1 to the second voltage mode.

Next, the control of the switching control mode 2 is described. When thecontrol unit 101 detects in S107 that the STANBY signal has reached alow-level state, the control unit 101 changes a mode from the secondvoltage mode to the switching control mode 2. In S209, the control unit101 prohibits the intermittent control when moving to the switchingcontrol mode 2. In S210, the control unit 101 determines whether or notthe turn-on duty of the FET1 is larger than MinDuty. When the controlunit 101 determines in S210 that the turn-on duty of the FET1 is largerthan MinDuty, the control unit 101 advances the flow to S108. In S108,the control unit 101 performs control of gradually decreasing theturn-on duty of the FET1, and continues the switching control mode 2until the control unit 101 detects the FB terminal voltage of Vth5 ormore in S109.

When the control unit 101 determines in S210 that the turn-on duty ofthe FET1 is MinDuty (predetermined turn-on duty) or less, the controlunit 101 ends the switching control mode 2 even when the FB terminalvoltage has not reached Vth5 or more, and advances the flow to S211.After the control unit 101 allows the intermittent control in S211, thecontrol unit 101 returns the flow to S101, and transitions to the firstvoltage mode.

Through repetition of the above-mentioned control, the control unit 101performs switching control of the voltage Vout of the switching powersupply 100. The control method for the switching power supply 100 in thesecond embodiment has the following features in addition to the featuresof the control method described in the first embodiment.

-   -   The control unit 101 performs the intermittent control in order        to improve the power supply efficiency when the load of the        voltage Vout is further low in the first voltage mode.    -   The control unit 101 has Step 1 of elongating the cycle in the        switching control mode 1.    -   The control unit 101 sets the prohibition period of the        intermittent control in the switching control mode 2.

Therefore, through use of the control method in the second embodiment,overshoot and undershoot of the output from the switching power supplycan be prevented, and further a through-current can be prevented fromflowing through the switching element, with the result that thereliability of the switching power supply can be enhanced. As describedabove, according to the second embodiment, when the output voltage ofthe switching power supply is controlled to be changed, overshoot andundershoot can be prevented, and a through-current can be prevented fromflowing through the switching element.

The switching power supply 100, which is the power supply apparatusdescribed in the first and second embodiments, is applicable, forexample, as a low-voltage power supply of an image forming apparatus,that is, a power supply configured to supply electric power to acontroller (control unit), a drive unit, for example, a motor, and thelike. A configuration of an image forming apparatus to which the powersupply apparatus according to the first and second embodiments isapplied is described below.

[Configuration of Image Forming Apparatus]

As an example of an image forming apparatus, a laser beam printer isexemplified. A schematic configuration of a laser beam printer as anexample of an electrophotographic printer is illustrated in FIG. 10. Alaser beam printer 300 includes a photosensitive drum 311 serving as animage bearing member on which an electrostatic latent image is formed, acharging portion 317 (charging unit) configured to uniformly charge thephotosensitive drum 311, and a developing portion 312 (developing unit)configured to develop the electrostatic latent image formed on thephotosensitive drum 311 with a toner. A toner image developed to thephotosensitive drum 311 is transferred onto a sheet (not shown) servingas a recording material supplied from a cassette 316 with a transferportion 318 (transfer unit). The toner image transferred onto the sheetis fixed with a fixing device 314, and the sheet is delivered to a tray315. The photosensitive drum 311, the charging portion 317, thedeveloping portion 312, and the transfer portion 318 form an imageforming unit. The laser beam printer 300 further includes a power supplyapparatus 400 (switching power supply 100) described in the first andsecond embodiments. The image forming apparatus to which the powersupply apparatus 400 according to the first and second embodiments isapplicable is not limited to the laser beam printer 300 illustrated inFIG. 10, and may be, for example, an image forming apparatus including aplurality of image forming units. The image forming apparatus mayfurther include a primary transfer portion configured to transfer thetoner image on the photosensitive drum 311 onto an intermediate transferbelt and a secondary transfer portion configured to transfer the tonerimage on the intermediate transfer belt onto the sheet.

The laser beam printer 300 includes a controller 320 configured tocontrol an image forming operation by the image forming unit and a sheetconveying operation, and the power supply apparatus 400 described in thefirst and second embodiments supplies electric power to the controller320, for example. Further, the power supply apparatus 400 described inthe first and second embodiments supplies electric power to a driveunit, for example, a motor for rotating the photosensitive drum 311 ordriving various rollers configured to convey the sheet. That is, theload in the first and second embodiments corresponds to the controller320 or the drive unit.

When the image forming apparatus according to a third embodiment of thepresent invention is in a standby state (for example, a power savingmode or a standby mode) for achieving power saving, power consumptioncan be lowered by reducing the load by, for example, supplying electricpower only to the controller 320. That is, at a time of the power savingmode, the image forming apparatus of the third embodiment performs theintermittent control at a time when the power supply apparatus 400 is alight load state described in the second embodiment. Further, the STANBYsignal input to the feedback unit 116 and the switching notificationunit 118 in the first and second embodiments may be a signal output fromthe controller 320.

As described above, according to the third embodiment, it is possible toprevent overshoot and undershoot and prevent a through-current fromflowing through a switching element when an output voltage of aswitching power supply is controlled to be changed.

While the present invention has been described with reference toexemplary embodiments, it is to be understood that the invention is notlimited to the disclosed exemplary embodiments. The scope of thefollowing claims is to be accorded the broadest interpretation so as toencompass all such modifications and equivalent structures andfunctions.

This application claims the benefit of Japanese Patent Application No.2017-090175, filed Apr. 28, 2017, which is hereby incorporated byreference herein in its entirety.

What is claimed is:
 1. A power supply apparatus comprising: atransformer including a primary winding and a secondary winding; a firstswitching element connected in series to the primary winding of thetransformer; a second switching element connected in parallel to theprimary winding of the transformer; a capacitor connected in series tothe second switching element, and is connected in parallel to theprimary winding of the transformer together with the second switchingelement; a smoothing and rectifying unit configured to smooth andrectify a voltage induced to the secondary winding of the transformer; afeedback unit configured to output a signal according to the voltagesmoothed and rectified by the smoothing and rectifying unit; a controlunit configured to control an operation of the first switching elementand the second switching element based on the signal output from thefeedback unit so that the voltage smoothed and rectified by thesmoothing and rectifying unit becomes a predetermined voltage, thecontrol unit configured to implement a control operation in a firstvoltage state, in which the predetermined voltage is a first voltage,and in a second voltage state, in which the predetermined voltage is asecond voltage larger than the first voltage; and a notification unitconfigured to notify the control unit of switching between the firstvoltage state and the second voltage state, wherein the control unit isconfigured to perform one of the following: when the notification unitnotifies switching from the first voltage state to the second voltagestate, the control unit changes a condition to a first switchingcondition of performing control of gradually increasing a turn-on dutyof the first switching element, and changes a voltage state to thesecond voltage state after a level of the signal output from thefeedback unit reaches a first value or less, and when the notificationunit notifies switching from the second voltage state to the firstvoltage state, the control unit changes a condition to a secondswitching condition of performing control of gradually decreasing theturn-on duty of the first switching element, and changes a voltage stateto the first voltage state after the level of the signal output from thefeedback unit reaches a second value or more; and when the notificationunit notifies switching from the first voltage state to the secondvoltage state by, the control unit changes a condition to the firstswitching condition of performing control of gradually increasing theturn-on duty of the first switching element, and changes a voltage stateto the second voltage state after the level of the signal output fromthe feedback unit reaches the first value or more, and when thenotification unit notifies switching from the second voltage state tothe first voltage state, the control unit changes a condition to thesecond switching condition of performing control of gradually decreasingthe turn-on duty of the first switching element, and changes a voltagestate to the first voltage state after the level of the signal outputfrom the feedback unit reaches the second value or less.
 2. A powersupply apparatus according to claim 1, wherein when the notificationunit notifies switching to the first voltage state after moving to thefirst switching condition, the control unit changes a voltage state tothe first voltage state after moving to the second switching conditionbefore moving to the second voltage state.
 3. A power supply apparatusaccording to claim 1, wherein when the notification unit notifiesswitching to the second voltage state after moving to the secondswitching condition, the control unit changes a voltage state to thesecond voltage state after moving to the first switching conditionbefore moving to the first voltage state.
 4. A power supply apparatusaccording to claim 1, further comprising a detection unit configured todetect a state of a load supplied with the voltage smoothed andrectified by the smoothing and rectifying unit, wherein the control unitis capable of controlling, in the first voltage state, an intermittentoperation of repeating a first period and a second period in accordancewith the state of the load detected by the detection unit, the firstperiod is a period in which a switching operation of alternately turningon and off the first switching element and the second switching elementis performed with a dead time for turning off both the first switchingelement and the second switching element being interposed, and thesecond period is a period in which the switching operation is halted,and wherein when the notification unit notifies switching from thesecond voltage state to the first voltage state by the notificationunit, the control unit changes a condition to the second switchingcondition after prohibiting the intermittent operation, and allows theintermittent operation after changing a condition from the secondswitching condition to the first voltage state.
 5. A power supplyapparatus according to claim 4, wherein the control unit is configuredto change a voltage state to the first voltage state when the turn-onduty of the first switching element reaches a predetermined turn-on dutyor less in the second switching condition.
 6. A power supply apparatusaccording to claim 4, wherein the control unit is configured to performa continuous operation of repeating the first period in the secondvoltage state to perform first control of gradually elongating a cycleof the switching operation by gradually elongating a turn-on time periodof the first switching element in the first switching condition, andthereafter perform second control of gradually shortening the cycle ofthe switching operation by gradually shortening a turn-on time period ofthe second switching element.
 7. A power supply apparatus according toclaim 6, wherein the control unit is configured to perform the secondcontrol after the turn-on time period of the first switching elementreaches a predetermined time period or more during a period in which thefirst control is performed.
 8. A power supply apparatus according toclaim 6, wherein in the first voltage state, the control unit isconfigured to perform the intermittent operation when the level of thesignal becomes lower than a first level, and to perform the continuousoperation when the level of the signal becomes higher than a secondlevel.
 9. A power supply apparatus according to claim 4, wherein thecontrol unit is configured to vary, based on the state of the loaddetected by the detection unit, a speed at which the turn-on duty of thefirst switching element in one of the first switching condition and thesecond switching condition is changed.
 10. A power supply apparatusaccording to claim 4, wherein the control unit is configured to set, inone of the first switching condition and the second switching condition,a speed at which the turn-on duty of the first switching element in alatter half of control is changed to be lower than a speed at which theturn-on duty of the first switching element in a former half of thecontrol is changed.
 11. A power supply apparatus according to claim 4,wherein the control unit is configured to determine the turn-on duty ofthe first switching element and a cycle of the switching operation basedon the signal output from the feedback unit, and wherein, when the levelof the signal is the same, the turn-on duty of the first switchingelement in the first voltage state is lower than the turn-on duty of thefirst switching element in the second voltage state, and the turn-ontime period of the first switching element in the first voltage state isshorter than the turn-on time period of the first switching element inthe second voltage state.
 12. An image forming apparatus comprising: animage forming unit configured to form an image; and a power supplyapparatus configured to supply electric power to the image formingapparatus, the power supply apparatus including: a transformer includinga primary winding and a secondary winding; a first switching elementconnected in series to the primary winding of the transformer; a secondswitching element connected in parallel to the primary winding of thetransformer; a capacitor connected in series to the second switchingelement, and is connected in parallel to the primary winding of thetransformer together with the second switching element; a smoothing andrectifying unit configured to smooth and rectify a voltage induced tothe secondary winding of the transformer; a feedback unit configured tooutput a signal according to the voltage smoothed and rectified by thesmoothing and rectifying unit; a control unit configured to control anoperation of the first switching element and the second switchingelement based on the signal output from the feedback unit so that thevoltage smoothed and rectified by the smoothing and rectifying unitbecomes a predetermined voltage, the control unit configured toimplement a control operation in a first voltage state, in which thepredetermined voltage is a first voltage, and in a second voltage state,in which the predetermined voltage is a second voltage larger than thefirst voltage; and a notification unit, which is configured to notifythe control unit of switching between the first voltage state and thesecond voltage state, wherein the control unit is configured to performone of the following: when the notification unit notifies switching fromthe first voltage state to the second voltage state, the control unitchanges a condition to a first switching condition of performing controlof gradually increasing a turn-on duty of the first switching element,and changes a voltage state to the second voltage state after a level ofthe signal output from the feedback unit reaches a first value or less,and when the notification unit notifies switching from the secondvoltage state to the first voltage state, the control unit changes avoltage state to a second switching condition of performing control ofgradually decreasing the turn-on duty of the first switching element,and changes a voltage state to the first voltage state after the levelof the signal output from the feedback unit reaches a second value ormore; and when the notification unit notifies switching from the firstvoltage state to the second voltage state, the control unit changes acondition to the first switching condition of performing control ofgradually increasing the turn-on duty of the first switching element,and changes a voltage state to the second voltage state after the levelof the signal output from the feedback unit reaches the first value ormore, and when the notification unit notifies switching from the secondvoltage state to the first voltage state, the control unit changes acondition to the second switching condition of performing control ofgradually decreasing the turn-on duty of the first switching element,and changes a voltage state to the first voltage state after the levelof the signal output from the feedback unit reaches the second value orless.
 13. An image forming apparatus according to claim 12, furthercomprising: a controller configured to control an operation of theimaging forming unit; and a drive unit configured to drive the imageforming unit, wherein the power supply apparatus is configured to supplyelectric power to one of the controller and the drive unit.
 14. A powersupply apparatus comprising: a transformer including a primary windingand a secondary winding; a first switching element connected in seriesto the primary winding of the transformer; a second switching elementconnected in parallel to the primary winding of the transformer; acapacitor connected in series to the second switching element, andconnected in parallel to the primary winding of the transformer togetherwith the second switching element; a smoothing and rectifying unitconfigured to smooth and rectify a voltage induced to the secondarywinding of the transformer; a feedback unit configured to output asignal according to the voltage smoothed and rectified by the smoothingand rectifying unit; and a control unit configured to control anoperation of the first switching element and the second switchingelement based on the signal output from the feedback unit so that thevoltage smoothed and rectified by the smoothing and rectifying unit is apredetermined voltage, wherein the control is configured to implement acontrol operation in a first voltage state, in which the predeterminedvoltage is a first voltage, and in a second voltage state, in which thepredetermined voltage is a second voltage larger than the first voltage,and wherein the control unit is configured to perform control ofgradually changing a turn-on duty of the first switching element in oneof a case in which the first voltage state is switched to the secondvoltage state and a case in which the second voltage state is switchedto the first voltage state.
 15. A power supply apparatus according toclaim 14, further comprising a notification unit configured to notifythe control unit of switching between the first voltage state and thesecond voltage state, wherein the notification unit is configured tonotify the control unit of one of switching from the first voltage stateto the second voltage state and switching from the second voltage stateto the first voltage state.
 16. A power supply apparatus according toclaim 14, wherein the control unit is configured to perform control ofgradually increasing the turn-on duty of the first switching elementwhen the first voltage state is switched to the second voltage state,and wherein the control unit is configured to perform control ofgradually decreasing the turn-on duty of the first switching elementwhen the second voltage state is switched to the first voltage state.17. An image forming apparatus comprising: an image forming unitconfigured to form an image; and a power supply apparatus configured tosupply electric power to the image forming apparatus, the power supplyapparatus including: a transformer including a primary winding and asecondary winding; a first switching element connected in series to theprimary winding of the transformer; a second switching element connectedin parallel to the primary winding of the transformer; a capacitorconnected in series to the second switching element, and connected inparallel to the primary winding of the transformer together with thesecond switching element; a smoothing and rectifying unit configured tosmooth and rectify a voltage induced to the secondary winding of thetransformer; a feedback unit configured to output a signal according tothe voltage smoothed and rectified by the smoothing and rectifying unit;and a control unit configured to control an operation of the firstswitching element and the second switching element the first switchingelement and the second switching element based on the signal output fromthe feedback unit so that the voltage smoothed and rectified by thesmoothing and rectifying unit is a predetermined voltage, wherein thecontrol unit is configured to implement a control operation in a firstvoltage state, in which the predetermined voltage is a first voltage,and in a second voltage state, in which the predetermined voltage is asecond voltage larger than the first voltage, and wherein the controlunit is configured to perform control of gradually changing a turn-onduty of the first switching element in one of a case in which the firstvoltage state is switched to the second voltage state and a case inwhich the second voltage state is switched to the first voltage state.18. An image forming apparatus according to claim 17, furthercomprising: a controller configured to control an operation of theimaging forming unit; and a drive unit configured to drive the imageforming unit, wherein the power supply apparatus is configured to supplyelectric power to one of the controller and the drive unit.